1. Field of the Invention
The present invention relates to an imaging apparatus, such as a video camera, having a function of optically correcting a shake caused by a camera shake to prevent blurring of an image picked up by the imaging apparatus.
2. Description of the Related Art
In an imaging apparatus such as a video camera, automatization and multifunctionalization have been achieved in all points such as an automatic exposure (AE) and automatic focus (AF) so that good imaging can be easily performed.
In recent years, a shake applied to the imaging apparatus has been a main cause of deteriorating the quality of a picked-up image, as the imaging apparatus has been reduced in size and the magnification of an optical system has been increased. Various types of image stabilization (camera shake correction) functions for improving an image blur in the picked-up image caused by the shake applied to the imaging apparatus have been proposed. Such an image stabilization function is mounted on the imaging apparatus, so that better imaging can be easily performed.
An example of the imaging apparatus having the image stabilization function is a camera having a configuration illustrated in FIG. 6.
In an imaging apparatus 200 having an image stabilizing function, an angular velocity sensor 201 is attached to an imaging apparatus main body, and detects a shake applied to the imaging apparatus 200 as an angular velocity. A direct current (DC) cut filter 202 cuts off a DC component of an angular velocity signal output from the angular velocity sensor 201, and passes only an alternate current (AC) component, i.e., a vibration component.
An amplifier 203 amplifies the angular velocity signal that has passed through the DC cut filter 202 to have suitable sensitivity, and outputs the amplified angular velocity signal. An analog-to-digital (A/D) converter 204 digitizes the angular velocity signal output from the amplifier 203, and outputs the digitized angular velocity signal.
A central processing (CPU) unit 223, for example, functions as a high pass filter (HPF) 205, an integrator 206, a pan/tilt determination unit 222, a control filter 208, a pulse width modulation unit 209, a focal length correction unit 207, and a motion vector processing unit 221.
The HPF 205 has a function of changing cut-off frequency characteristics in any frequency band, and cuts off a low frequency component included in the digitized angular velocity signal (angular velocity data) output from the A/D converter 204, and outputs the angular velocity data. The integrator 206 has a function of changing frequency characteristics in any frequency band, and integrates the angular velocity data output from the HPF 205, and outputs the result of the integration as angular displacement data.
The focal length correction unit 207 acquires current zoom position information from a zoom encoder 217 for detecting a zoom position in an imaging optical system 213 for performing zooming and focusing operations, calculates a focal length from the information, and calculates a driving amount (gyro-based correction data) of a correction optical system 212 from information relating to the focal length and the above-mentioned angular displacement data.
In the angular velocity sensor 201 using a vibration gyro or the like, angular velocity detection properties are degraded in a low frequency of 1 Hz or less. In this low frequency band, therefore, the effect of a correction error becomes significant. As a result, an uncorrected image shake in a low frequency band cannot be corrected sufficiently, and the quality of the image is deteriorated.
The imaging apparatus 200 further includes a unit for detecting the remaining image-shake of the picked-up image in addition to detecting an angular velocity, i.e., a motion vector detection unit 220, which detects the remaining image-shake in the low frequency band and corrects the remaining image-shake, to improve correction performance as described below.
The motion vector detection unit 220 detects, from information relating to the picked-up image obtained by an image sensor 218 in the imaging apparatus 200, a motion vector of the image based on a luminance signal included in a video signal generated by a signal processing unit 219. The signal processing unit 219 generates a video signal conforming to a national television system committee (NTSC) format, for example. The motion vector processing unit 221 converts the motion vector detected by the motion vector detection unit 220 into a driving amount (vector-based correction data) of the correction optical system 212.
The vector-based correction data is a signal for correcting the remaining image-shake in the low frequency band. The vector-based correction data added to the above-mentioned gyro-based correction data becomes a final driving amount (final correction data) of the correction optical system 212 for making shake correction in the whole frequency band from the low frequency band to a high frequency band.
A difference between the final correction data and a value (position detection data) obtained by digitizing an output of a position detection sensor 214 for detecting the position of the correction optical system 212 in an A/D converter 216 is input to the control filter 208. The pulse width modulation unit 209 converts an output of the control filter 208 into a pulse width modulation (PWM) signal and outputs the PWM signal.
A motor driving unit 215 drives a motor 211 for moving the correction optical system 212 based on the PWM signal from the pulse width modulation unit 209, and changes an optical axis of light incident on an imaging surface, to optically correct an image-shake of the picked-up image.
The pan/tilt determination unit 222 determines panning/tilting based on the angular velocity data output from the A/D converter 204 and the angular displacement data output from the integrator 206, to carry out panning control. More specifically, if the angular velocity data is equal to a predetermined threshold value or more, or the angular displacement data (the result of the integration) is equal to a predetermined threshold value or more even if the angular velocity data is less than the predetermined threshold value, the pan/tilt determination unit 222 determines that the imaging apparatus 200 is in a panning state or a tilting state, to carry out panning control.
In the panning control, first the low cutoff frequency of the HPF 205 is shifted to the higher frequency side. Thus, shake correction does not respond to a low frequency. Furthermore, a time constant used for an integration operation in the integrator 206 is shifted in a direction to decrease the value thereof.
Thus, a shake correction position is gradually moved toward the center of a movement range, so that the angular displacement data output from the integrator 206 gradually comes closer to a reference value (a value that can be taken with no shake applied to the imaging apparatus 200). Furthermore, a gain in calculating the vector-based correction data from the detected motion vector in the motion vector processing unit 221 is shifted in a direction to decrease the value thereof.
On the other hand, unless the angular velocity data is equal to a predetermined threshold value or more, or the angular displacement data is equal to a predetermined threshold value or more even if the angular velocity data is less than the predetermined threshold value, the pan/tilt determination unit 222 determines that the imaging apparatus 200 is not in a panning state or a tilting state, to shift the low cutoff frequency of the HPF 205 to the lower frequency side, also shift the time constant used for the integration operation in the integrator 206 in a direction to increase the value thereof, and further shift the gain in calculating the vector-based correction data from the detected motion vector in the motion vector processing unit 221 in a direction to increase the value thereof.
This causes the low cutoff frequency of the HPF 205, the time constant used for the integration operation in the integrator 206, and the gain in calculating the vector-based correction data in the motion vector processing unit 221 to return to their respective original states, to cancel the panning control.
The above-mentioned control carried out by the pan/tilt determination unit 207 is discussed in Japanese Patent Application Laid-Open No. 11-187308, which is effective as shake correction control for the panning state or the tilting state because image-shake correction in a high frequency band can be performed while suppressing image-shake correction in a low frequency band. However, in the conventional image stabilizing function as in the imaging apparatus 200, however, the following problems exist.
FIG. 7A is a graph illustrating a change of an output of the angular velocity sensor 201 from the start to the end of a panning operation (angle-of-view changing operation). In FIG. 7A, the panning operation is gradually accelerated from time T1 to time T2, is performed at a predetermined speed from time T2 to time T4, and is gradually decelerated from time T4 to time T5, to end.
FIG. 7B is a graph illustrating a change of an output (angular velocity data) of the A/D converter 204 during the panning operation described above. FIG. 7C is a graph illustrating a change of an output (angular displacement data) of the integrator 206 during the panning operation described above.
The change of the angular velocity data illustrated in FIG. 7B is a change of an output of the angular velocity sensor 201 after passing through the DC cut filter 202, so that a DC component of the angular velocity data is attenuated from time T2 to time T4. A threshold value at which the transition to panning control for the angular velocity data occurs is set to Pan_th1, as illustrated in the graph of FIG. 7B. When the angular velocity data exceeds Pan_th1 from time T2 to time T3, the panning control is started.
Because of the effect of the attenuation of the DC component, it is not determined that the panning operation is being performed even during the panning operation from time T3 to time T4. T3 and T3′ may be any time between time T2 and time T4, and may change with the speed and the time of panning.
A threshold value at which the transition to panning control for the angular displacement data is set to Pan_th2, as illustrated in FIG. 7C. When the angular displacement data exceeds Pan_th2 from time T2 to time T3, it is determined that the panning operation (angle-of-view changing operation) is started, and the panning control is started.
When the panning control is started, the low cutoff frequency of the HPF 205 is shifted to the higher frequency side, and the time constant used for the integration operation in the integrator 206 is shifted in a direction to decrease the value thereof, as described above. Even if the angular velocity data is greatly shifted toward the pulse side from time T2 to time T3 illustrated in FIG. 7B, therefore, the angular displacement data is restrained from increasing, to gradually come closer to a reference value (a value that can be taken with no shake applied to the imaging apparatus 200).
As a result, the angular displacement data gradually comes closer to the reference value from time T3 to time T3′. When the angular displacement data is below Pan_th2, it is determined that the panning operation (angle-of-view changing operation) is terminated, and the panning control is canceled.
When the panning control is canceled, the low cutoff frequency of the HPF 205 is shifted to the lower frequency side, and the time constant used for the integration operation in the integrator 206 is also shifted in a direction as to increase the value thereof, as described above.
If the angular displacement data is shifted toward the pulse side, as illustrated in FIG. 7B, therefore, the angular displacement data increases to exceed Pan_th2 again, the panning control is started. From time T3 to time T3′, the transition to the panning control and the cancel of the panning control are thus repeated, resulting in unnatural moving of the picked-up image.
From time T3′ to time T4, the DC component of the angular velocity data is converged to zero, as illustrated in FIG. 7B. Therefore, the angular displacement data does not greatly vary, so that it is not determined that the panning operation is being performed.
From time T4 to time T5, the angular velocity data varies in the minus direction by the output of the angular velocity sensor 201 at the time when the panning operation is decelerated. At time T4, it is not determined that the panning operation is being performed, as described above. Therefore, the low cutoff frequency of the HPF 205 is set lower, and the time constant used for the integration operation in the integrator 206 is also set longer.
When the angular velocity data varies in the minus direction, therefore, its signal component is not attenuated. Therefore, the angular displacement data greatly varies in the minus direction. As a result, the picked-up image moves, although a user does not move the imaging apparatus 200, after the panning operation is terminated.
As described above, in the conventional image stabilizing function, the low frequency component during the panning operation is attenuated by the DC cut filter 202, so that the panning control is canceled even if the panning operation is being performed. Therefore, the picked-up image becomes an unnatural image.